1. Field of the Invention
The present invention relates to an angular rate sensor having a vibrator that is supported by a substrate in a floated manner. In particular, the invention relates to an angular rate sensor in which a floating semiconductor thin film that has been formed by a semiconductor micromachining technique is driven in the x-direction by electrically causing attraction for the thin film and its cancellation by using interdigitated comb electrodes, though the invention is not limited to such a case.
2. Description of the Related Art
In typical angular rate sensors of the above type, a floating thin film has one set of floating comb finger electrodes at each of its left-side and right-side positions (left-side floating comb finger electrodes and right-side floating comb finger electrodes). Two sets of fixed comb finger electrodes are also provided. That is, left-side fixed comb finger electrodes and right-side fixed comb finger electrodes each of which are interdigitated, in a parallel manner, with those of each corresponding set of floating comb electrodes so as not to contact the latter. The floating thin film is vibrated in the x-direction by applying voltages alternately between the left-side floating comb finger electrodes and the left-side fixed comb finger electrodes and between the right-side floating comb finger electrodes and the right-side fixed comb finger electrodes. When the floating thin film has an angular rate of rotation about the z-axis, it receives a Coriolis force and comes to vibrate also in the y-direction (elliptical vibration). Where the floating thin film is a conductor or is formed by joining electrodes and a detection electrode is provided on a substrate so as to be parallel with the xz-plane of the floating thin film, the capacitance between the detection electrode and the floating thin film oscillates so as to correspond to the y-component (angular rate component) of the elliptical vibration. The angular rate can be determined by measuring a variation (in amplitude) of the capacitance. Reference is made to Japanese Unexamined Patent Publication Nos. Hei. 9-127148 and Hei. 9-42973 and Japanese Patent Application Nos. Hei. 8-249822 and Hei. 9-121989.
Conventionally, in the above type of angular rate sensor using a semiconductor thin film as a floating body (floating thin film), interdigitated comb finger electrodes or parallel plate electrodes are used for driving the floating body electrostatically in the x-direction that is parallel with a substrate or for detecting a displacement through capacitance. Those electrodes use a surface (xz-plane) formed by etching the semiconductor thin film.
From the principle of angular rate detection, the floating body needs to be movable in both x and y-directions. Therefore, conventionally, the floating body is made movable in both x and y-directions and supported by the substrate. The driving in the x-direction of the floating body is performed by parallel plate electrodes (floating comb finger electrodes and fixed comb finger electrodes) extending in the x-direction, and the parallel plate electrodes tend to exert attractive force in the y-direction on the floating body. One floating comb finger electrode extending in the x-direction is interposed, via gaps, between two fixed comb finger electrodes extending in the x-direction. If the one floating comb finger electrode is correctly located at the center of the gap between the two fixed comb finger electrodes, the two fixed comb finger electrodes exert electrostatic attractive forces, respectively, in the y-direction having the same absolute value and opposite directions to the one floating comb finger electrode and hence the fixed comb finger electrodes do not apply any driving force to the floating comb finger electrodes. However, if the one floating comb finger electrode is deviated even slightly toward one of the two fixed comb finger electrodes from the center of the gap between the two fixed comb finger electrodes, the fixed comb finger electrodes apply driving force in the y-direction to the floating comb finger electrodes even if the floating body does not have an angular rate about the z-axis and the floating body is thereby moved in the y-direction.
Having the same period as a displacement that occurs when the floating body has an angular rate, this movement may cause an offset that results in an event that a detection signal of the same kind as would be obtained when an angular rate exists is generated when there exists no angular rate. This movement may also cause an offset variation due to a temperature variation (i.e., a temperature drift) and hence tends to lower the accuracy of angular rate detection.
Further, since the floating body needs to be movable in both x and y-directions, the spring constant in the z-direction of a structure for supporting the floating body with respect to the substrate tends to be small. If the spring constant is small, the relative position in the z-direction of the floating comb finger electrodes with respect to the fixed comb finger electrodes is deviated by a displacement in the z-direction of the floating body due to acceleration in the z-direction and the capacitance between those electrodes is thereby varied. A variation in the capacitance acts as a disturbance for the detection of an angular rate about the z-axis. Therefore, it is preferable to prevent a displacement of the floating body in the z-direction that is caused by acceleration in the z-direction.
FIG. 22 shows an example of a conventional angular rate sensor. Floating body anchors 305, electrode pads for fixed electrodes, and an electrode pad 315 for vibration detection electrodes, all of which are made of polysilicon containing an impurity for imparting conductivity (hereinafter referred to as conductive polysilicon), are joined to a silicon substrate 301 that is formed with an insulating layer. A floating body 302 and fixed electrodes 308, which are semiconductor thin films made of conductive polysilicon, are connected to connection electrodes via wiring lines 312 that are formed on the insulating layer on the silicon substrate 301. Each floating body anchor 305 is continuous with floating support beams 318 extending in the x-direction, which are continuous with respective floating support beams 304 extending in the y-direction. The floating support beams 304 are continuous with the rectangular-ring-like floating body 302 that is substantially parallel with the surface of the substrate 301.
A plurality of moving-side comb finger electrodes 306 for x-driving, which are arranged in the y-direction at a constant pitch, project from the floating body 302 leftward and rightward in the x-direction. Each fixed electrode 308 has fixed comb finger electrodes 307 for x-driving that are inserted in interfinger slots of the moving-side comb finger electrodes 306. Very small gaps exist between the moving-side comb finger electrodes 306 for x-driving and the fixed comb finger electrodes 307 for x-driving.
A number of electrodes 309 for y-displacement detection that extend in the x-direction are arranged in the y-direction on the floating body 302. One of many fixed electrodes 310 of a first set for y-displacement detection and one of many fixed electrodes 311 of a second set for y-displacement detection are located in each one-pitch gap of the electrodes 309. The fixed electrodes 310 of the first set are connected to a wiring line 314 that is formed on the insulating layer on the silicon substrate 301, and the fixed electrodes 311 of the second set are connected to the other wiring line that is similar to the wiring line 314 and is formed on the insulating layer on the silicon substrate 301.
The floating support beams 304 and 318, the floating body 302, and the comb fingers 307 of the fixed electrodes 308 are separated from the surface of the substrate 301 in the z-direction. That is, the former are opposed to the surface of the substrate 301 with a gap in between. The floating support beams 304 and 318, the floating body 302, and the comb fingers 307 of the fixed electrodes 308 are formed so as to be continuous with and integral with the floating body anchors 305 and the electrode pads for the fixed electrodes 308 after the floating body anchors 305 and the electrode pads for the fixed electrodes 308 have been formed on the surface of the silicon substrate 301 by a micromachining technique. In this specification, a supporting mode in which, as described above, a body is separated from the surface of the substrate 301 in the z-direction and can be displaced or bent in the x and/or y-direction with respect to the substrate 301 is called "floating" or "movable."
Since the floating support beams 304 for supporting the floating body 302 are floated above the substrate 301 and extend in the y-direction, they are not bent in the y-direction but are prone to be bent in the x-direction. Therefore, the floating body 302 is prone to vibrate in the x-direction. Further, since the floating support beams 318 that are continuous with the floating support beams 304 are floated above the substrate 301 and extend in the x-direction, they are not bent in the x-direction but are prone to be bent in the y-direction. Therefore, the floating body is prone to vibrate in the y-direction, too.
The floating body 302 is connected to an x-driving circuit (not shown) via the floating support beams 304 and 318 and a wiring line 313 and is connected to the apparatus ground (GND) there. The two fixed electrodes 308 are connected to the x-driving circuit via the respective wiring lines 312 and electrode pads. The x-driving circuit applies high voltages alternately to the two fixed electrodes 308 and repeats this operation. The floating body 302 is attracted rightward (as viewed in FIG. 22) when a high voltage is applied to the right-side fixed electrode 308 and is attracted leftward when a high voltage is applied to the left-side fixed electrode 308, whereby the floating body 302 is vibrated in the x-direction.
When the floating body 302 has an angular rate of rotation about the z-axis, a Coriolis force is exerted on the floating body 302 and is thereby vibrated also in the y-direction (elliptical vibration). When the floating body 302 is moved in the +y direction, the distance (or the capacitance) between the movable electrodes 309 of the floating body 302 and the fixed electrodes 311 decreases (or increases) and the distance (or the capacitance) between the movable electrodes 309 of the floating body 302 and the fixed electrodes 310 increases (or decreases). When the floating body 302 is moved in the -y direction, the distance varies in the opposite manner. While the movable electrodes 309 are kept at the apparatus ground (GND) potential, the fixed electrodes 310 and 311 are connected to a capacitance detection circuit (not shown). The capacitance detection circuit generates an electrical signal that represents a difference between the capacitances between the movable electrodes 309 and the fixed electrodes 310 and 311. The level of the electrical signal corresponds to the amplitude of vibration of the floating body 302 in the y-direction. A signal processing circuit (not shown) converts the electrical signal into a signal representing an angular rate (i.e., an angular rate signal).
From the principle of angular rate detection, the floating body needs to be movable in both x and y-directions. Therefore, conventionally, the floating body is made movable in both x and y-directions and supported by the substrate. Incidentally, the driving in the x-direction of the floating body is performed by the parallel plate electrodes (floating comb finger electrodes and fixed comb finger electrodes) extending in the x-direction, and the parallel plate electrodes tend to exert attractive force in the y-direction on the floating body. One floating comb finger electrode extending in the x-direction is interposed, via gaps, between two fixed comb finger electrodes extending in the x-direction. If the one floating comb finger electrode is correctly located at the center of the gap between the two fixed comb finger electrodes, the two fixed comb finger electrodes exert electrostatic attractive forces, respectively, in the y-direction having the same absolute value and opposite directions to the one floating comb finger electrode and hence the fixed comb finger electrodes do not apply any driving force to the floating comb finger electrodes. However, if the one of the floating comb finger electrodes is deviated even slightly toward one of the two fixed comb finger electrodes from the center of the gap between the two fixed comb finger electrodes, the fixed comb finger electrodes apply driving force in the y-direction to the floating comb finger electrodes even if the floating body does not have an angular rate about the z-axis and the floating body is thereby moved in the y-direction.
Having the same period as a displacement that occurs when the floating body has an angular rate, this movement may cause an offset that results in an event that a detection signal of the same kind as would be obtained when an angular rate exists is generated when there exists no angular rate. This movement may also cause an offset variation due to a temperature variation (i.e., a temperature drift) and hence tends to lower the accuracy of angular rate detection.
Further, since the floating body needs to be movable in both x and y-directions, the spring constant in the z-direction of a structure for supporting the floating body with respect to the substrate tends to be small. If the spring constant is small, the relative position in the z-direction of the floating electrodes with respect to the fixed electrodes is deviated by a displacement in the z-direction of the floating body due to acceleration in the z-direction and the capacitance between those electrodes is thereby varied. A variation in the capacitance acts as a disturbance for the detection of an angular rate about the z-axis. Therefore, it is preferable to prevent a displacement of the floating body in the z-direction that is caused by acceleration in the z-direction.
Further, since a displacement of the movable portion due to a Coriolis force is small, a detection signal is small, as a result of which high-precision detection circuits (i.e., the above-mentioned capacitance detection circuit and signal processing circuit) are required. On the other hand, if it is intended to increase a detection signal, the sensor becomes larger and the material costs increase.